I bought and used a TIM (Telecom Italia Mobile) sim card for my unlocked iPhone 4S while in Italy. It was a big help with the following: communicating (voice & text messages) with apartment owners where we stayed; getting suggestions for close-by restaurants and wine bars; downloading tourist apps (e.g., Firenze app); navigating while walking in Roma, Firenze, Venezia; and navigating while driving from Venezia to the Dolomite mountains.
We arrived FCO (Rome), trained into the city on the "Leonardo Express" to Termini station where TIM has a store. I described my anticipated usage over the 2 week stay and the TIM salesperson recommended a pay as you go with an initial 1 GB data allowance. During 2 weeks we used approximately 50% of the 1 GB by utilizing wifi when at our apartments to reduced the data usage. Cost was €27. The salesperson installed and tested the connection before leaving the store - we were up and running immediately. You'll need your passport and 20 minutes or so, as well as your euros/credit card.